Conventionally, a powder-weighing apparatus shown in FIG. 4 has been used to weigh powder. This powder-weighing apparatus includes a weighing hopper 13 for retaining powder. Powder is charged from a charging device 15 to this weighing hopper 13, and a load cell 12 measures the weight of the powder retained inside the weighing hopper 13. As such a powder-weighing apparatus, a weighing apparatus having a two-stage charge system has been employed to improve its weighing speed and weighing accuracy. FIG. 5 shows a relationship between charge amounts, charge flow rates, and charge times in the two-stage charge system. As shown in FIG. 5, in a weighing apparatus having a two-stage charge system, for the purpose of increasing its weighing speed, powder is charged into a weighing hopper at a high flow rate until a weighed value of the powder reaches approximately 90% of a target weight (100%) (quick-charge step), and the remaining 10% of the powder is charged into the weighing hopper at a low flow rate (slow-charge step).
In such a weighing apparatus having a two-stage charge system, ways to further increase its weighing speed while maintaining its weighing accuracy are, for example, (1) to further increase a powder amount to be charged in a quick-charge step from the above-mentioned approximately 90%, or (2) to increase a flow rate of powder to be weighed in a quick-charge step. However, in (1), where a powder amount to be charged in a quick-charge step is increased, overshooting takes place during the changeover from a powder flow rate in the quick-charge step to a powder flow rate in the slow-charge step, resulting in a situation in which the slow-charge step is not executed because the target weight is assumed to be reached. In this case, a final weight of the powder to be obtained is far below the target weight. Similarly, in (2), where a powder flow rate in a quick-charge step is increased, overshooting takes place during the changeover from a powder flow rate in a quick-charge step to a powder flow rate in a slow-charge step, thereby resulting in a situation in which a final weight of the powder to be obtained is below the target weight. In addition, since a controller is necessary to change over from a quick-charge step to a slow-charge step, there is such a drawback that the cost of the weighing apparatus becomes high. Furthermore, since the time-consuming slow-charge step must be executed to ensure the weighing accuracy, there is such a problem that the weighing speed can not be increased above a certain level.
In order to solve the shortcomings of a weighing apparatus having a two-stage charge system, weighing apparatuses having a stageless charge system have been developed. FIG. 7 shows a relationship among charge amounts, charge flow rates, and charge times in a stageless charge system. In the stageless charge system, for the purpose of avoiding overshooting during the changeover from a powder flow rate in a quick-charge step to a powder flow rate in a slow-charge step, a powder charge flow rate is controlled so as to continuously decrease from a powder flow rate in the quick-charge step to a powder flow rate in the slow-charge step based on a predetermined function. However, realization of such a weighing apparatus having a stageless charge system requires a controller for controlling difficult stageless changeover from a powder flow rate in a quick-charge step to a powder flow rate in a slow-charge step, thereby increasing the cost of the weighing apparatus. Furthermore, similarly to the two-stage charge system described above, a time-consuming slow-charge step must be executed to ensure its weighing accuracy, and therefore there is such a problem that its weighing speed can not be increased above a certain level. The maximum performance of a weighing apparatus of this system is approximately 12–15 bags/min.